Kotlin DSL - union structure

I am designing a DSL and run into a requirement where I have a variable which could be assigned to different ways. Greatly simplified, I would like to set value property either by an integer or by an expression in String. (The real need is even more complex.)

I would like to write in my DSL:

value = 42

value = "6*7"

Behind the scene, the value will be stored in a DynamicValue<Int> structure which contains either an integer or the expression.

class DynamicValue<T>(dv : T?, expr : String) {
    val directValue : T? = dv
    val script : String? = expr
    ...
}

I tried several ways (delegate, class, etc), but none of them provided these syntax.

Is there a way to declare this union like structure?

Solution

What do you think about the following syntax:

value(42)
value("6*7")
//or
value+=42
value+="6*7"

You can do this with operator functions:

class DynamicValue<T>() {
    var dv: T? = null
    var expr: String? = null

    operator fun invoke(dv : T)  {
        this.dv = dv
        this.expr = null
    }

    operator fun invoke(expr: String)  {
        this.dv = null
        this.expr = expr
    }

    operator fun plusAssign(dv : T)  {
        this.dv = dv
        this.expr = null
    }

    operator fun plusAssign(expr: String)  {
        this.dv = null
        this.expr = expr
    }
}

You can't redefine the assign operator in Kotlin, therefor the pure syntax value=42 is not possible.

But I wouldn't go with operator functions, it's to magical. I would do this:

val value = DynamicValue<Int>()
value.simple=42
value.expr="6*7"

class DynamicValue2<T>() {
    private var _dv: T? = null
    private var _expr: String? = null
    var simple: T?
        get() = _dv
        set(value) {
            _dv = value
            _expr = null
        }

    var expr: String?
        get() = _expr
        set(value) {
            _expr = value
            _dv = null
        }
}